1. Field of the Invention
This invention relates to a process for producing graphite blocks which have utility as X-ray and neutron ray monochromators, neutron ray filters and the like radiation optical devices. The invention also relates to a process for carbonizing graphitizable polymer films.
2. Description of the Prior Art
Graphite has an important position as industrial materials because of its outstanding heat and chemical resistances and high electric conductivity, and has been widely used as gaskets, electrodes, heating elements and structural materials. Especially, highly oriented graphite has good spectral and reflective characteristics and has been thus used as X-ray or neutron ray monochromators, filters and the like. Natural graphite may be used for such purposes. Natural graphite with high quality occurs in an extremely limited amount and is intractable because of its powder or flaky form. Therefore, efforts of producing artificial graphite have been heretofore made.
One of the production processes of artificial graphite is one which includes pyrogenic deposition of hydrocarbons in a vapor phase and hot working of gaseous hydrocarbons. In the process, re-annealing is effected at a temperature of 3400.degree. C. for a long time under pressure. Graphite thus obtained is called highly oriented pyrographite (HOPG) and has almost the same properties as those of natural graphite. However, this process has the disadvantage that the manufacturing process is very complicated with a low yield, so that the production costs become very high.
In order to solve the problems involved in the above process and to produce graphite easily and inexpensively, processes for producing high-quality graphite by heating polymer films have been developed and proposed. Organic polymers usually belong to non-graphitizable materials. It has been generally accepted that these non-graphitizable materials are not converted into high-quality graphite when heated to high temperatures, for example, of 3000.degree. C. Recent studies made by us have revealed that some kinds of polymer materials can be converted to graphite of high quality by appropriately controlled thermal treatment. It was found that the polymers which are graphitizable include, for example, polyoxadiazole, aromatic polyimides, aromatic polyamides, polybenzoimidazole, polybenzobisthiazole, polybenzooxazole, polythiazole, poly-p-phenylenevinylene and the like.
Based on this finding, we proposed graphitization of organic polymers, for example, in JP-A- 61-275114, 61-275115 and 61-275117.
On the other hand, fabrication of graphite blocks by hot pressing a plurality of superposed graphitizable polymer films has also been proposed in JP-A- 1-105199 and 63-235218.
However, graphite or graphite blocks, which are obtained by the procedures described in these laid-open applications, are not necessarily satisfactory. For instance, when a plurality of graphitizable polymer films are merely placed between a pair of substrates, caulked by means of bolts and thermally treated as set forth in JP-A-1-105199, there is not obtained a highly oriented graphite block. For obtaining a highly oriented graphite block, inner layers of the graphite should have regularly aligned crystals which are highly oriented. In addition, the respective layers should be strongly bonded together. The thermal treatment by mere application of pressure will result in a film which is wrinkled or involves an internal strain. In the worst case, the film will be broken. Thus, it is very difficult to obtain a graphite block having good properties.